1. Field of the Invention
The present invention relates to an image-processing apparatus and an image-processing method for generating record data used in an image-forming apparatus that forms an image by performing recording scans a plurality of times on the same image region on a recording medium.
2. Description of the Related Art
An inkjet printer is known as an example of an image-forming apparatus that forms an image by performing recording scans a plurality of times on the same image region on a recording medium. In an inkjet printer, an image is printed by discharging ink droplets from a recording head and landing them on a recording medium while reciprocally moving the recording head in a main-scanning direction (performing a recording scan) and transporting the recording medium in a sub-scanning direction. An inkjet printer visually expresses a wide variety of colors by forming several colors of very small dots in one region. The dot arrangement is determined through a quantization process performed by a driver. The dot arrangement significantly affects the image quality, and, thus, various techniques have been conventionally proposed in order to improve the arrangement.
Japanese Patent Laid-Open No. 8-279920 (Patent Document 1) discloses a technique for uniformly distributing dots by performing quantization with coordination between different colors. In this technique, the quantization process is performed by comparing the sum of input values and a threshold value. For example, in the case where blue is expressed by selecting cyan and magenta as different colors, if cyan and magenta are subjected to error diffusion independently of each other, a portion may be formed in which dots are arranged too close to or too away from each other, and the pattern may become visually uncomfortable (reference numeral 901 in FIG. 9). However, if the error diffusion is performed in a coordinated manner, a uniform dot arrangement is obtained in which cyan dots and magenta dots are dispersed from each other, and a visually preferable result is obtained (reference numeral 902 in FIG. 9).
Furthermore, Japanese Patent Laid-Open No. 60-107975 (Patent Document 2) uses a multi-pass recording method in which recording scans (overprint operations) are performed a plurality of times in the same region, because, when an image is printed, non-uniformity occurs in the orientation, the size, the dot landing position, and the like of ink droplets due to errors caused by physical factors such as the properties of each nozzle, errors in the paper-transporting amount, differences in the movement amount of the recording head, and the like. According to this method, the above-described non-uniformity of dots is evened out, and the image quality can be improved. It is known that, if dots on record data are arranged so as to be dispersed from each other between recording scans at that time, deterioration in the image quality due to non-uniformity of the dots is actually further suppressed on a paper face.
Here, as described above, in one region of an image, an inkjet printer performs image formation using a plurality of colors, or by performing recording scans a plurality of times using one color, and, in many cases, dots are preferably arranged so as to be dispersed from each other between colors or between recording scans. The reason for this is as follows: in the case where non-uniformity occurs in the orientation, the size, the dot landing position, and the like of ink droplets, if ink dots placed in one region are overlapped on each other, or close to each other but not overlapped, a color change or a darkness change on a paper face varies, and, even if dots are not overlapped on each other, darkness non-uniformity tends to visually appear when dots are arranged too close to each other.
A color change or a darkness change, which is visually uncomfortable, can be suppressed by generating data in advance in which dots are arranged so as to be dispersed from each other, thereby preventing the dots from being overlapped on each other or being too close to each other on a paper face. In particular, in a highlight portion with a low darkness, a color change or a darkness change is visually conspicuous, and, thus, dots are preferably dispersed in the highlight portion.
Furthermore, as another technique, Japanese Patent Laid-Open No. 2003-116015 (Patent Document 3) discloses a technique for obtaining quantized output values using a table. Furthermore, Japanese Patent Laid-Open No. 2008-188805 (Patent Document 4) discloses a technique for dispersing dots by obtaining frequency components of an image, and letting the components pass through a low-pass filter.
However, it is difficult to apply the techniques disclosed in Patent Documents 1 to 4 to any number of colors or any number of recording scans. If the number of colors or the number of recording scans increases, in conditional branch methods, branches for determining output values are complicated, and, thus, the control circuit is complicated. Furthermore, in table methods, the table data size increases, and, thus, a large memory is necessary, and the cost increases.